This invention relates to a power transmission device for use in a hybrid car, a recently developed car having a second driving source, i.e. an electric motor besides the conventional internal combustion engine, and more particularly to a power transmission device for selectively connecting the engine and/or the motor to the drive train.
The motor and the engine of a hydrid car are automatically connected to and disconnected from the drive train according to their respective revolving speeds so as to improve the total fuel economy. Generally, while the car is accelerating, the motor delivers power to the traction wheels, and while the car is decelerating, the motor serves as a generator by being driven by the traction wheels. Electric power thus generated is stored for later use.
A clutch is needed to automatically connect and disconnect the motor and the engine to and from the drive train. Such a clutch has to be one that is automatically controlled according to the state of car.
FIG. 17A shows a typical drive train arrangement of hybrid cars, in which the internal combustion engine A (hereinafter simply "engine") and the motor B have their output shafts arranged in series. In this arrangement, when the car starts or while it is traveling at a low speed, only the motor B is used to drive the car, while the engine A is at a stop. To keep the engine A at a stop, its output shaft is disconnected from the drive train by disengaging an electromagnetic clutch or brake C. Another electromagnetic clutch or brake C is used to disconnect the motor B from the transmission D to keep the transmission D neutral.
FIG. 17B shows another drive train layout of a hybrid car, which is disclosed e.g. in unexamined Japanese patent publication 9-95149. In this arrangement, the motor B is connected to the rear propeller shaft, while the engine A is connected to the transmission D. Thus, not only the engine power delivered through the transmission D but also the motor power are transmitted to the front vehicle wheels through the front differential and the front wheel axles.
In this arrangement, the engine A is rotating at all times, and the motor B assists the engine while the car is accelerating. While it is decelerating, the traction wheels drive the motor for regeneration. The motor B is connected to the rear propeller shaft through an electromagnetic clutch or brake C.
The above-mentioned conventional electromagnetic clutches or brakes C comprise an electromagnet and friction disks or brake shoes. In order to transmit the power of the engine A and the motor B, large friction disks and a powerful electromagnet are needed.
The electromagnet consumes much power and produces much heat because it has to be kept activated to keep the engine A and/or the motor B coupled to the drive train.
Also, friction disks and brake shoes tend to wear quickly and cause slippage. They are thus short-lived.
An object of this invention is to provide a power transmission device suited for use in a hybrid car for selective transmission of its engine and motor powers.